Hydrogen is a revolutionary new type of fuel that has emerged quite recently. As a fuel, hydrogen is abundant, affordable, clean and renewable. The only product of hydrogen reaction with oxygen is water that is not polluting. Hydrogen can be produced from renewable sources, and with nil carbon foot print. Hydrogen fuel cells, which produce electrical power form hydrogen, offer several advantages over petroleum-based internal combustion engines, including: water vapor emission, high efficiency, quiet operation, low friction and high energy to weight ratio.
Nonetheless, current production, storage and distribution methods of hydrogen are a significant impediment to the widespread use of hydrogen as an alternative energy source. The most common method of hydrogen distribution involves producing hydrogen gas, liquefying or pressurizing the hydrogen into a pressurized cylinder, shipping the cylinders to the point of use, and releasing the hydrogen from the cylinders. Hydrogen is flammable over a wide range of concentrations in air, and low spark temperatures. Thus, storage, distribution, and use of hydrogen in tanks have to be highly regulated and controlled. Hydrogen tanks are often heavy, contain specialized explosion-proof components, and thus expensive, due to the need to provide the necessary safety measures. Nevertheless, even with these precautions, there is still a significant risk that hydrogen may be released, and explode during loading, unloading, distribution or use, such as accidents and vandalism. Such risks render an unfavorable approach towards powering motorized vehicles with hydrogen.
The predicaments associated with liquid hydrogen, have raised the interest in the use of hydrides as suitable means for storing hydrogen fuel. The term hydride is widely applied to describe compounds of hydrogen with other elements.
The quantity of hydrogen which can be stored per cubic centimeter can be higher in a hydride than in liquid hydrogen.
However, recharging of hydrogen storage device takes generally a long period of time, which can be the order of several hours. Due to this long recharging period, the usefulness of hydrides for hydrogen storage devices has been limited, as has their use in heat engines and non-mechanical compressors and refrigerators, which are even more dependent than storage devices for their utility upon having a very fast heat transfer rate.
The initial activation of hydride-formers by a process of bonding hydrogen atoms with other elements (“hydriding”) can take several days, during which the particles crumble into smaller pieces. Nevertheless, the length of time for a subsequent re-hydriding process is more of interest since it specifies the time of charging a hydrogen storage apparatus and out letting the hydrogen from the apparatus. It should be noted that the rate of charging and out letting of the apparatus might be slow unless the heat transfer rate of the apparatus is sufficiently fast. A high heat transfer rate can be provided by using metal compounds for the hydride apparatus. As a metal absorbs hydrogen, it expands and internal stresses cause it to fracture and break apart into smaller pieces (“decrepitation”). The particle size is reduced each absorption and desorption cycle until, eventually, the particles disintegrate into a submicron-sized powder (“fines”). The hydride fines compact conduct heat poorly and do not readily allow hydrogen to permeate. Consequently, hydrogen absorbance deteriorates each successive cycle.
If the powder becomes entrained in the gas stream, it can migrate and contaminate downstream piping and equipment. Even if filters are used to contain the powder, the fine mesh required for such small particles gets easily clogged.
Low absorption and desorption rates can be also caused by limitations due to slow solid state diffusion of the hydrogen in the solid particles, oxide and other coating barriers on the particle surface.
Another problem of hydrogen storage in hydride beds is the volume change of the particles which is associated with absorption and desorption of hydrogen. Since hydrides lose volume up to 20% when releasing hydrogen, the powder tends to collect at the bottom of the container, and when hydrogen is absorbed, the powder expands, and exerts forces on the container, and when this process is repeated many times, damage can be accrued to the container, limiting its life cycle length.
The long time taken to recharge hydride bed containers is mainly due to the limited coefficient of heat conductivity of hydride powders. Since all the processes for absorption and desorption of hydrogen in solids involve certain latent heat that needs to be released (on absorption) or supplied (on desorption), the hydrogen refueling and release depend critically on the heat conductance of the hydride material and its containment system.
Numerous attempts have been made in the art to provide fast and stable hydrides for hydrogen storage and a number of endeavors have been made to alleviate these problems.
U.S. Pat. No. 4,249,654 titled “Hydrogen storage apparatus” discloses a hydrogen storage container, having at least one valved port that is filled to about 75% of capacity with particles of low density material having a hydride forming metal coated on the surface of the particles by vapor or vacuum deposition. The density of the particle is on the order of 5% to 50% of the density of the metal coating thereon. Hydrogen gas is adsorbed into or released from the lattice structure of the hydride forming metal.
The art includes also hydrides that are combined with stable, non-hydridable matrix materials to form compositions that are better able to withstand repeated absorption and desorption cycles than hydrides alone.
U.S. Pat. No. 4,717,629 title “Compact of hydrogen absorption alloy” discloses a compact of hydrogen adsorption alloy principally composed of a metal hydride in which all surfaces of fine particles of hydrogen adsorption alloy are completely coated with a dissimilar metal by plating, without effecting reactivity and a porous material of high thermal conductivity is infiltrated with the fine particles of alloy to be formed into a compact by compression molding.
The poor heat transfer of powdered metal hydrides has been recognized as an impediment for a fast absorption and desorption apparatus. One solution was to form an aggregate constructed as a highly porous metal skeleton with metal hydride consolidated therein.
U.S. Pat. No. 4,607,826 titled “Apparatus for preparing improved porous metal-hydride compacts” discloses a method and apparatus for preparing metal-embedded porous metallic-hydride (“PMH”) compacts capable of withstanding repeated hydriding-dehydriding cycles without disintegrating. The finely divided hydridable metal alloy hydride is admixed with a finely divided metal selected from Al, Ni, Cu or other transition metals and charged with hydrogen. The resulting mixture is sintered in a furnace in which hydrogen is introduced at a pressure above the equilibrium pressure to the prevailing temperature, mechanical stress being applied simultaneously. The compacts obtained possess outstanding stability, as shown by the fact that they have remained intact even after more than 6000 cycles.
Numerous hydride compounds, which have substantial stability, absorption and desorption rates, are disclosed in the art. Thus what is needed, and not provided by the art, is a way to integrate those compounds into thermally efficient containers.